The present invention relates to a steam turbine having brush seals between non-rotatable and rotatable components arranged and located to eliminate thermal bowing resulting from non-uniform distribution of heat about the rotatable component due to frictional contact between the brush seal and the rotatable component and particularly relates to apparatus and methods for eliminating axial thrust loads in the event of failure of the brush seal in such turbine.
In U.S. Pat. No. 6,168,377, of common assignee herewith, there is disclosed a steam turbine having a brush seal located between a non-rotatable component and a rotatable component of the rotor shaft. Particularly, axial flanges are provided on the dovetails of the buckets, the bucket dovetails being secured in complementary fashion to the dovetail of a rotor wheel. A brush seal comprised of an arcuate array of metal bristles projecting from the non-rotatable component toward the rotatable component, i.e., the flanges on the bucket dovetails, has bristle tips engaging with and bearing against the flange surfaces. As will be appreciated from a review of that patent, the contact between the bristles of the brush seal and the opposing sealing surface, i.e., the flanges, generates heat.
As disclosed in that patent, it is recognized that the contact between the brush seal and the sealing surface should be located radially outwardly of the rotor shaft in order to isolate the generated heat from the outer diameter of the rotor. Otherwise, the friction-generated heat may cause a non-uniform temperature distribution about the circumference of the shaft, resulting in non-uniform axial expansion of the rotor and, hence, a bow in the rotor. While various methods and apparatus are disclosed in that patent for eliminating that problem, one such solution locates the friction-generating surface on the bucket dovetail flanges radially outboard of the outer shaft diameter. In that manner, the generated heat is isolated from the rotor, eliminating any tendency of the rotor to bow.
That patented design and other designs utilize conventional labyrinth-type packing seals on the inside of the diaphragm web as a backup to the brush seal. These labyrinth seals are located directly adjacent the outer diameter of the shaft. Brush seals are, however, susceptible to wear and failure. Should a brush seal spaced outwardly from the shaft fail, e.g., the brush seal of that patented design, the sealing diameter changes from the bucket dovetail platform to the rotor shaft. This, in turn, adversely changes the pressure distribution on the shaft and the thrust on the rotor in an axial direction. Accordingly, there is a need to provide a sealing system for a steam turbine in which not only is the problem of thermal bowing of the steam turbine rotor due to non-uniform heat distribution resulting from contact between brush seals and complementary sealing surfaces eliminated, but also the axial thrust loads on the rotor bearings are eliminated or minimized in the event of brush seal failure.
In accordance with a preferred embodiment of the present invention, there is provided a brush seal located radially outwardly of the outer diameter of the shaft of the rotatable component to eliminate thermal bowing of the rotor due to non-uniform heat distribution in combination with a labyrinth seal at substantially the same radial location to eliminate thrust loads in the event of failure of the brush seal. To accomplish the foregoing, the rotatable component, i.e., the rotor shaft, has an annular rim or platform projecting radially outwardly into the wheelspace between axially adjacent wheels. The platform is in the form an annular pedestal having a neck and a flange or fin extending axially toward the axially opposed wheel at the radially outer extremity of the pedestal. It will be appreciated that the platform extends into the wheelspace defined between the axially adjacent wheels and the stationary component.
The flange or fin on the platform has an annular sealing surface for engagement by the bristle tips of an annular brush seal carried by the web of the diaphragm. With the flange or fin thus cantilevered in an axial direction and defining the sealing surface, it will be appreciated that for heat generated by frictional contact between the bristles and the sealing surface to affect rotor dynamics, the heat must traverse first axially along the flange or fin and then radially inwardly toward the rotor. This geometry enables the heat generated by the frictional contact of the bristles on the sealing surface to be substantially dissipated or dissipated to the extent that rotor dynamics are not affected by any heat generated by contact between the brush seal and sealing surface.
Additionally, a labyrinth seal is also provided in the wheelspace. Particularly, the upstream or downstream buckets have flanges which project into the wheelspace in an axial direction. In radial registration with the bucket flanges are one or more labyrinth seal teeth carried by the surrounding annular web. These labyrinth seal teeth cooperable with the bucket dovetails are provided to mitigate steam turbine section performance degradation in the event of brush seal failure. The labyrinth teeth carried by the web may also lie on the upstream or downstream sides or on both sides of the brush seal and cooperate with the rotor platform flange to provide the backup seal. The matched radial location of the brush and labyrinth seals about the rotor wheels also mitigates the effect on rotor thrust in the event of brush seal failure. As a consequence, the foregoing described design enables application of brush seals to all stages of the diaphragm packing area within current brush seal application limitations.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotatable component including a rotor shaft and a non-rotatable component about the rotatable component, a brush seal carried by the non-rotatable component for sealing engagement with the rotatable component, first and second wheels on the rotatable component spaced axially from one another, the rotatable component including a plurality of buckets spaced circumferentially from one another on each of the wheels, means for inhibiting non-uniform circumferential heat transfer to the rotatable component thereby to eliminate or minimize bow of the rotatable component due to frictional contact between the brush seal and the rotatable component, the inhibiting means including an annular platform projecting radially outwardly of an outer surface of and from the rotor shaft at an axial location between the first and second wheels, flanges extending axially from the buckets on the second wheel in a direction toward the platform and the first wheel and spaced radially outwardly of the outer surface of the rotor shaft, the brush seal disposed between the buckets and engaging a sealing surface on the platform radially outwardly of the outer surface and at least one labyrinth seal tooth extending between the stationary component and the bucket flanges.
In a further preferred embodiment according to the present invention, there is provided in a steam turbine having a rotatable component including a rotor shaft mounting axially spaced buckets and a non-rotatable component about the rotatable component carrying a brush seal for sealing engagement with the rotatable component, a method of substantially eliminating bowing of the rotor resulting from circumferentially non-uniform distribution of heat about the rotatable component due to frictional contact between the brush seal and the rotatable component comprising the steps of inhibiting circumferential non-uniform heat transfer to the rotatable component resulting from heat generated by frictional contact between the rotatable component and the brush seal by locating the brush seal radially outwardly of the rotor shaft for sealing a steam leakage path between the rotatable and non-rotatable components, providing upstream directed flanges on the buckets downstream of the brush seal and radially outwardly of the rotor shaft and locating a labyrinth seal on the non-rotatable component for sealing cooperation with a sealing surface on the flanges at substantially the same radial distance from the shaft as the brush seal.